S4 Cant hear contacts

When I make or receive a call my phone won't let me hear the person on the other end and they can't hear me.
I have tried turning off and on the phone, it will work for about 2 minutes (if that long) and then it will act up again.
I've tried to restore my phone but it still does the same thing.
I've made no recent changes, I actually just bought the phone 2 days ago.
My wife and I have the same phones, the settings on both are exactly the same, but hers worls correctly.
Any help would be appreciated.
Samsung Galaxy S4 i545l Androis version 4.3